Microscopes are optical instruments used to examine minute specimens by presenting an enlarged, well-resolved image of the specimen for the observer. For purposes of the present invention, the term "specimen" includes living tissue and cells to be observed via a microscope under physiological conditions. A compound microscope includes an objective lens and an eye piece mounted in a tube with a variable apparatus for directing and concentrating light on the specimen.
In order to accurately observe living specimens under a microscope, the physiological conditions required to maintain the viability of the specimen should be duplicated. For example, physiological conditions for living cells generally means keeping the cells at a physiological temperature, i.e., between approximately 32.degree. and 38.degree. C. At these temperatures, cells will remain healthy and retain physiological activity, i.e., normal metabolism, motility and growth.
Several methods have been used in the prior art to maintain a particular specimen at a required physiological temperature when the specimen is being viewed under a microscope. One of the earliest methods placed the microscope and the operator in a "warm room," i.e., a room in which the thermostat had been set at the desired physiological temperature. Under these conditions, the specimen could be observed at the desired temperature. However, there are several drawbacks to the use of a warm room. First, not everyone has access to a warm room facility. The size of the room and its heat control create a substantial investment in location and cost. Another disadvantage is that the user is also subject to "warm room" conditions, which can be uncomfortable over a long period of time. Further, the temperature conditions cannot be rapidly changed. For example, if the user wanted to change the conditions from 38.degree. C. to 32.degree. C., all of the equipment in the room would have to be moved, cooled and re-equilibrated to the new temperature. This process could take several hours and would have to be done under dry conditions to avoid condensation on the optic lenses. Other problems, such as subjecting photographic film and/or electronic cameras to elevated heat are also apparent.
Another specimen heating system includes the use of stage heaters. Stage heaters work on the principle of heating the specimen in a dish such as a petri dish, in which the tissue culture is growing. The stage heater is designed to heat the sides and lower rim of the dish or chamber. Reference is made to U.S. Pat. No. 4,629,862 to Kitagawa et al. and U.S. Pat. No. 4,888,463 to Middlebrook for a description of microscope stage heaters. The limitations to stage heaters include the size of the device. As with other temperature controlled approaches, stage heaters often take up a great deal of room on the stage limiting access for research manipulation. This is especially true for water-jacket stage heaters, which require water lines entering the stage heater area.
Another disadvantage with stage heaters is that they have difficulty adjusting for changes in temperature gradient. This limitation is especially apparent with microscopes employing oil immersion lenses. Oil immersion lenses are designed to touch the specimen dish, slide or plate. A drop of lens oil is placed between the lens and the specimen dish. By touching the specimen dish, the lenses become a heat sink drawing heat from the specimen. Thus, although the culture dish containing the specimen may be heated, the area actually being observed, i.e., the area adjacent the objective lens, may be 3 to 12.degree. cooler than the rest of the specimen dish due to the contact between the cooler objective lens and the specimen dish. Therefore, an effective specimen heating system must heat not only the specimen dish but also the objective lens of the microscope.
One approach to heating both the specimen dish and the objective lens is by infrared radiation with a thermistor-controlled heat lamp. Typically, a lamp is placed above the stage to warm the sample and stage area and another lamp is placed below the stage to heat the objective lens and the bottom of the specimen container. However, because heat is provided with a lamp, it is difficult to maintain a constant temperature. Even with thermistor control, the lamp or lamps tend to cycle on and off. The heat-absorbing surfaces of the microscope will expand and contract with the cycling. This condition subjects the specimens to varied temperature changes over time. It also affects the focus of the optical system causing focus drift and instability. Additionally most lamps are bulky and crowd the area around the microscope. Further, the user must be careful to avoid placing obstructing items in the path of the infrared light. Further still, some cameras used to photograph samples are infrared sensitive. This is specifically a problem in immunolabelling techniques, which require maximum signal-to-noise ratios at minimal fluorescence levels.
Another technique is the use of objective lens heaters. In situations involving small volumes of media, i.e., generally 2 milliliters (ml) or less, used in conjunction with oil-immersion objective lenses, the simplest and most efficient means of keeping cells at a physiological temperature may be to heat the objective lens and limit evaporation with either a sealed specimen chamber or an overlay of oil. For larger volumes of media, an objective heater can be used in conjunction with a stage heater. One method of heating an objective lens is to wrap the lens with a flexible coil of tubing and run water through it at an appropriate temperature. Although this method may accomplish the purpose of heating the objective lens, it is inefficient as it involves adding water lines and expensive water heater and circulator means to maintain a constant temperature. There is also a risk of water damage if lines should leak or break.
Accordingly, it is a principal object of the present invention to provide an improved system for heating a specimen being observed with a microscope.
It is further an object of the present invention to provide an objective lens heating system, which is simple to use, efficient and avoids the risk of damage to the microscope and/or the specimen.
It is yet another object of the present invention to provide a system for maintaining the physiological temperature of a specimen being observed under a microscope, which prevents the objective lens from drawing heat from the specimen dish.